1. Field of the Invention
The present invention relates to a vibration isolation apparatus provided between a vibration producing portion and a vibration receiving portion in machinery such as vehicles, general industrial machinery and so on, to absorb vibration from the vibration producing portion.
2. Description of the Related Art
A vibration isolation apparatus is used in a vehicle as an engine mount between an engine and a vehicle body for absorbing vibration generated by the engine. This type of vibration isolation apparatus is equipped with a plurality of fluid chambers which can expand and contract and are connected by restricted paths. In the vibration isolation apparatus, vibration of the engine transmitted to one of the fluid chambers causes the fluid to be displaced into the other fluid chamber. The displacement of the fluid meets with a flow resistance so that the vibration is absorbed.
The engine vibrates in various modes of vibration. For example, a shake vibration is generated when the vehicle is traveling at speeds of about 70 to 80 km/hr, and an idle vibration is generated when the engine idles. Frequency range of the idle vibration is 20 to 40 Hz while that of the shake vibration is 8 to 15 Hz. Therefore, vibration frequencies at different vibration modes of the engine vary.
A conventional vibration isolation apparatus, however, can only be effective in a specific range of vibrational frequency which is determined by the sectional area and length of a restricted path of the device, and may not be effective for absorbing vibration in a frequency range other than the specific range. Therefore, the conventional vibration isolation apparatus does not effectively absorb idle vibration when the device is adjusted for effective absorption of shake vibration. Conversely, it does not effectively absorb shake vibration when the device is adjusted for effective absorption of idle vibration.